Rollup merge of #86843 - FabianWolff:issue-86820, r=lcnr

Check that const parameters of trait methods have compatible types

This PR fixes #86820. The problem is that this currently passes the type checker:
```rust
trait Tr {
    fn foo<const N: u8>(self) -> u8;
}

impl Tr for f32 {
    fn foo<const N: bool>(self) -> u8 { 42 }
}
```
i.e. the type checker fails to check whether const parameters in `impl` methods have the same type as the corresponding declaration in the trait. With my changes, I get, for the above code:
```
error[E0053]: method `foo` has an incompatible const parameter type for trait
 --> test.rs:6:18
  |
6 |     fn foo<const N: bool>(self) -> u8 { 42 }
  |                  ^
  |
note: the const parameter `N` has type `bool`, but the declaration in trait `Tr::foo` has type `u8`
 --> test.rs:2:18
  |
2 |     fn foo<const N: u8>(self) -> u8;
  |                  ^

error: aborting due to previous error
```
This fixes #86820, where an ICE happens later on because the trait method is declared with a const parameter of type `u8`, but the `impl` uses one of type `usize`:
> `expected int of size 8, but got size 1`

3 files changed, 106 insertions, 0 deletions

diff --git a/compiler/rustc_typeck/src/check/compare_method.rs b/compiler/rustc_typeck/src/check/compare_method.rs
index 12d0c14a3d5..d3586888155 100644
--- a/compiler/rustc_typeck/src/check/compare_method.rs
+++ b/compiler/rustc_typeck/src/check/compare_method.rs
@@ -66,6 +66,10 @@ crate fn compare_impl_method<'tcx>(
     {
         return;
     }
+
+    if let Err(ErrorReported) = compare_const_param_types(tcx, impl_m, trait_m, trait_item_span) {
+        return;
+    }
 }
 
 fn compare_predicate_entailment<'tcx>(
@@ -929,6 +933,68 @@ fn compare_synthetic_generics<'tcx>(
     if error_found { Err(ErrorReported) } else { Ok(()) }
 }
 
+fn compare_const_param_types<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    impl_m: &ty::AssocItem,
+    trait_m: &ty::AssocItem,
+    trait_item_span: Option<Span>,
+) -> Result<(), ErrorReported> {
+    let const_params_of = |def_id| {
+        tcx.generics_of(def_id).params.iter().filter_map(|param| match param.kind {
+            GenericParamDefKind::Const { .. } => Some(param.def_id),
+            _ => None,
+        })
+    };
+    let const_params_impl = const_params_of(impl_m.def_id);
+    let const_params_trait = const_params_of(trait_m.def_id);
+
+    for (const_param_impl, const_param_trait) in iter::zip(const_params_impl, const_params_trait) {
+        let impl_ty = tcx.type_of(const_param_impl);
+        let trait_ty = tcx.type_of(const_param_trait);
+        if impl_ty != trait_ty {
+            let (impl_span, impl_ident) = match tcx.hir().get_if_local(const_param_impl) {
+                Some(hir::Node::GenericParam(hir::GenericParam { span, name, .. })) => (
+                    span,
+                    match name {
+                        hir::ParamName::Plain(ident) => Some(ident),
+                        _ => None,
+                    },
+                ),
+                other => bug!(
+                    "expected GenericParam, found {:?}",
+                    other.map_or_else(|| "nothing".to_string(), |n| format!("{:?}", n))
+                ),
+            };
+            let trait_span = match tcx.hir().get_if_local(const_param_trait) {
+                Some(hir::Node::GenericParam(hir::GenericParam { span, .. })) => Some(span),
+                _ => None,
+            };
+            let mut err = struct_span_err!(
+                tcx.sess,
+                *impl_span,
+                E0053,
+                "method `{}` has an incompatible const parameter type for trait",
+                trait_m.ident
+            );
+            err.span_note(
+                trait_span.map_or_else(|| trait_item_span.unwrap_or(*impl_span), |span| *span),
+                &format!(
+                    "the const parameter{} has type `{}`, but the declaration \
+                              in trait `{}` has type `{}`",
+                    &impl_ident.map_or_else(|| "".to_string(), |ident| format!(" `{}`", ident)),
+                    impl_ty,
+                    tcx.def_path_str(trait_m.def_id),
+                    trait_ty
+                ),
+            );
+            err.emit();
+            return Err(ErrorReported);
+        }
+    }
+
+    Ok(())
+}
+
 crate fn compare_const_impl<'tcx>(
     tcx: TyCtxt<'tcx>,
     impl_c: &ty::AssocItem,
diff --git a/src/test/ui/const-generics/issue-86820.rs b/src/test/ui/const-generics/issue-86820.rs
new file mode 100644
index 00000000000..04650403c6b
--- /dev/null
+++ b/src/test/ui/const-generics/issue-86820.rs
@@ -0,0 +1,25 @@
+// Regression test for the ICE described in #86820.
+
+#![allow(unused,dead_code)]
+use std::ops::BitAnd;
+
+const C: fn() = || is_set();
+fn is_set() {
+    0xffu8.bit::<0>();
+}
+
+trait Bits {
+    fn bit<const I : u8>(self) -> bool;
+    //~^ NOTE: the const parameter `I` has type `usize`, but the declaration in trait `Bits::bit` has type `u8`
+}
+
+impl Bits for u8 {
+    fn bit<const I : usize>(self) -> bool {
+    //~^ ERROR: method `bit` has an incompatible const parameter type for trait [E0053]
+        let i = 1 << I;
+        let mask = u8::from(i);
+        mask & self == mask
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/const-generics/issue-86820.stderr b/src/test/ui/const-generics/issue-86820.stderr
new file mode 100644
index 00000000000..f4396f2f2b0
--- /dev/null
+++ b/src/test/ui/const-generics/issue-86820.stderr
@@ -0,0 +1,15 @@
+error[E0053]: method `bit` has an incompatible const parameter type for trait
+  --> $DIR/issue-86820.rs:17:18
+   |
+LL |     fn bit<const I : usize>(self) -> bool {
+   |                  ^
+   |
+note: the const parameter `I` has type `usize`, but the declaration in trait `Bits::bit` has type `u8`
+  --> $DIR/issue-86820.rs:12:18
+   |
+LL |     fn bit<const I : u8>(self) -> bool;
+   |                  ^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0053`.